"I have been warning throughout this campaign that North Korea is run by a lunatic who has been expanding his nuclear arsenal while President Obama has stood idly by," the GOP presidential candidate said in a statement.

The Florida senator, who has made national security and foreign policy a hallmark of his campaign, cast the test, if confirmed, as "just the latest example of the failed Obama-Clinton foreign policy."

"Our enemies around the world are taking advantage of Obama's weakness," Rubio said.

"We need new leadership that will stand up to people like Kim Jong-un and ensure our country has the capabilities necessary to keep America safe," he added.

Pyongyang said earlier in the day that it had successfully tested a hydrogen bomb as officials in South Korea and in the international community monitored seismic activity near a North Korean nuclear site.

The White House said that North Korea has conducted nuclear tests three times since 2006 and reiterated that the U.S. "will not accept it as a nuclear state."